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Modeling prevention interventions requires insights into epidemiological and biological factors affecting HIVtransmission. Project 1 of the proposed Program Project addresses the epidemiological determinants of HIVtransmission by examining the per contact transmission probability (P), the rate of partner change/ contactrate (c), and the duration of infectiousness (D), which all contribute to the reproductive number (R0= pcD),defined as the average number of secondary infections arising from a single infection in a completelysusceptible population. To achieve this objective, we will address the following specific aims: Aim 1 : Percontact transmission probability (P) - The per contact sexual transmission probability of HIV will beestimated, taking into account stage of HIV infection in the source partner, presence of viral and bacterialsexually transmitted infections, circumcision status, and other biological cofactors investigated in Project 2.The absolute and relative difference of transmission probability for insertive versus receptive anal sex willalso be considered. Aim 2 : Rate of partner change (c) - The role of partner selection in HIV acquisition andtransmission risk will be examined at individual, partnership, and network levels. At the individual level, therelative contribution of HIV risk due to number of sexual acts versus number of sexual partners will beexamined. At the partnership level, risk characteristics will be compared between transmitting andseroconcordant non-transmitting partnerships. At the network level, risk of acquisition and transmission ofHIV based on the location of individuals within networks of venues used to meet sex partners, andcorrelates of being in a transmission cluster, will be elucidated. Aim 3 : Duration of infectiousness (D) - Todetermine how duration of the infectious stages of HIV infection intersects with risk behaviors and time toantiretroviral treatment. We will use data from Aims 1-3 to develop a stochastic, individual-based model topredict the number of secondary infections arising from individuals diagnosed during recent HIV infection asthey transit to chronic infection, and the potential impact of short-course antiviral therapy on reducingtransmission. Project 1 depends on well-characterized clinical data and specimens collected by the Clinicaland Specimen Core and the subsequent measurement of transmission correlates in Project 2.Epidemiological data contributing to HIV transmission collected in Project 1 will help to define correlates ofHIV transmission in Project 2. The Administrative Core will coordinate administrative, fiscal, data, andstatistical support for Projects 1 and 2. These data will provide critical information on epidemiologicaldeterminants of HIV transmission in a setting where HIV-1 subtype B predominates, which are lacking forMSM, and will generate important exploratory data for other populations.